1. Field of the Invention
This invention relates to methods and apparatus for firing extruded metal structures. In particular, the invention relates to the firing of metal honeycombs (monoliths) for use as catalyst supports in vehicle exhaust systems.
2. Description of the Prior Art
As known in the art, the basic steps for creating an extruded metal structure comprise: 1) forming a mixture of one or more metal powders, an organic binder, and, as required, one or more additives, 2) extruding the mixture to form a green body, 3) drying the green body, 4) burning the binder out of the green body, and 5) sintering (densifying) the green body at temperatures above about 1150.degree. C. to produce the desired structure. See, for example, European Patent Publication No. 351,056 and U.S. Pat. Nos. 4,758,272 and 4,871,621, the relevant portions of which are incorporated herein by reference.
The present invention relates to steps (4) and (5), i.e., burn-out and sintering, which collectively will be referred to herein as "firing" of the green body. More particularly, the invention relates to methods and apparatus for controlling the environment around the green body during firing so as to 1) improve the sintering process, 2) reduce the level of contamination introduced into the metal structure during firing, and 3) improve the physical and chemical properties of the finished product.
As discussed in detail below, in accordance with the invention, it has been found that during the firing process, the metal powders of the green body are highly sensitive to even minute levels of contaminants, in particular, oxidative contaminants, which can react with the hot, and thus strongly reactive, powder. Such contaminants can arise from various sources including the furnace used for the firing, the gas or gases supplied to the furnace during firing (the "furnace gas" or the "processing gas"), or from the products produced upon burn-out of the binder (the "burn-out products"). The importance of protecting the green body from even minute levels of contaminants arising from such sources has not previously been recognized in the art. Similarly, the methods and apparatus discussed below which achieve the necessary levels of protection of the green body during firing have not been previously used in the art.
Various techniques for firing extruded metal structures have been disclosed in the art. For example, U.S. Pat. No. 2,902,363 discloses sintering a green body, composed of a mixture of a metal powder and an organic elastomer, in an atmosphere of hydrogen. See also U.S. Pat. No. 3,444,925 (argon or hydrogen) and U.S. Pat. No. 4,871,621 (argon or mixtures of nitrogen and hydrogen). Similarly, U.S. Pat. No. 2,709,651 discloses flowing a non-oxidizing gas such as hydrogen past a green body during firing. The flowing of the gas is said to aid in controlling the shrinkage of the green body as it is sintered. See also U.S. Pat. No. 4,758,272 (flowing argon) and EPO Patent Publication No. 351,056 (flowing hydrogen or a mixture of hydrogen and argon followed by flowing argon, hydrogen, or a mixture of hydrogen and argon).
Chemical means for improving the firing process have also been disclosed. In particular, U.S. Pat. No. 4,758,272 discloses including calcium or magnesium in the furnace to act as a getter for oxygen during firing. EPO Patent Publication No. 351,056 states that in place of calcium or magnesium, oxygen control can be achieved by burying the structure to be fired in fine or coarse alumina powder, by placing the structure on a zirconia plate, by burying the structure in zirconia beads, or by suspending the structure in a tapered alumina crucible.
Apparatus to aid in the drying of honeycomb structures is also known. Specifically, U.S. Pat. No. 4,439,929 discloses the use of a perforated support to hold ceramic green bodies during drying, while U.S. Pat. No. 4,837,943 discloses the perforated support in combination with a perforated cover (also referred to in the art as a "cookie").
Although these references address various aspects of the process of transforming extruded metals into rigid structures, none of them recognize or address the problem of protecting green bodies from minute levels of contaminants during firing.